Switched amplifier drive circuit for liquid crystal displays

ABSTRACT

A cell driver ( 70 ) for a display unit ( 50 ) having a memory element and a liquid crystal cell among a plurality of liquid crystal cells includes a first storage capacitor ( 14 ) and a first differential amplifier ( 16 ) selectively coupled between the first storage capacitor and the liquid crystal cell forming a first drive circuit. The cell driver also includes a second storage capacitor ( 14 ′) and a second differential amplifier ( 16 ′) coupled between the second storage capacitor and the liquid crystal cell forming a second drive circuit A switching mechanism ( 72  and  74 ) is used for switching the liquid crystal cell between the first and second drive circuits.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the field of video systems utilizing aliquid crystal display (LCD) or liquid crystal on silicon (LCOS), and inparticular, to a switched driver circuit for such displays.

[0003] 2. Description of Related Art

[0004] Liquid crystal on silicon (LCOS) can be thought of as one largeliquid crystal formed on a silicon wafer. The silicon wafer is dividedinto an incremental array of tiny plate electrodes. A tiny incrementalregion of the liquid crystal is influenced by the electric fieldgenerated by each tiny plate and the common plate. Each such tiny plateand corresponding liquid crystal region are together referred to as acell of the imager. Each cell corresponds to an individuallycontrollable pixel. A common plate electrode is disposed on the otherside of the liquid crystal (LC). The drive voltages are supplied toplate electrodes on each side of the LCOS array. Each cell, or pixel,remains lighted with the same intensity until the input signal ischanged, thus acting as a sample and hold. Each set of common andvariable plate electrodes forms an imager. One imager is provided foreach color, in this case, one imager each for red, green and blue.

[0005] It is typical to drive the imager of an LCOS display with aframe-doubled signal to avoid 30 Hz flicker, by sending first a normalframe in which the voltage at the electrodes associated with each cellis positive with respect to the voltage at the common electrode(positive picture) and then an inverted frame in which the voltage atthe electrodes associated with each cell is negative with respect thevoltage at the common electrode (negative picture) in response to agiven input picture. The generation of positive and negative picturesensures that each pixel will be written with a positive electric fieldfollowed by a negative electric field. The resulting drive field has azero DC component, which is necessary to avoid the image sticking, andultimately, permanent degradation of the imager. It has been determinedthat the human eye responds to the average value of the brightness ofthe pixels produced by these positive and negative pictures.

[0006] The present state of the art in LCOS requires the adjustment ofthe common-mode electrode voltage, denoted VITO, to be precisely betweenthe positive and negative field drive for the LCOS. The subscript ITOrefers to the material indium tin oxide. The average balance isnecessary in order to minimize flicker, as well as to prevent aphenomenon known as image sticking.

[0007] In the current art, the LCOS drive cell looks much like aconventional Active Matrix LCD driver. This does not work well, due tothe various artifacts discussed in the literature. The main causes areparasitic capacitance cross-talk, residual voltage in the LC cell, andvoltage droop of the LC, due to ionic leakage and bulk resistivity ofthe LC material. Mainly this has been solved by: 1. Increasing the cellcapacitance (limited by physical area), 2. Changing to higherresistivity LC materials (limits flexibility and response time), 3.Increasing the frame scan rate to more than 60Hz (expensive, and costsmore bandwidth). 4. Strongly controlling the temperature of the device,to maintain high voltage holding ratio (VHR).

[0008] The main cause for all of the above issues is that the availablecharge is only transferred to the LC cell once per frame. In a displaywith a million pixels, this limits the available power and doesn't allowfor any closed-loop check that the desired voltage has actually beenachieved on the pixel electrode. Additionally, the issues of flicker,drive voltage, and image retention have been addressed in various waysfor digital drive LCOS displays, but such methods fail to provide asolution to the problem in analog systems due to the need for continuousupdating. Thus, a need exists for a display driver that providesadequate isolation between a storage capacitor and a liquid crystal celland further eliminates flicker without the need for frame doubling.

BRIEF SUMMARY OF THE INVENTION

[0009] In a first aspect of the present invention, a display driver fora display unit having a memory and a liquid crystal cell among aplurality of liquid crystal displays comprises a first drive circuitcoupled to a first memory element of the liquid crystal cell, a seconddrive circuit coupled to a second memory element of the liquid crystalcell, and a switching arrangement for switching the liquid crystal cellbetween the first and the second drive circuits.

[0010] In a second aspect of the present invention, a display driveramong an array of array drivers for a display unit having acorresponding array of liquid crystal cells comprises a first storagecapacitance and a first amplifier selectively coupled between the firststorage capacitance and the liquid crystal cell forming a first drivecircuit, a second storage capacitance and a second amplifier coupledbetween the second storage capacitance and the liquid crystal cellforming a second drive circuit, and a switching arrangement forswitching the first and second drive circuits.

[0011] In a third aspect of the present invention, a method of driving aLCD/LCOS display comprises the steps of isolating a storage capacitancefrom a liquid crystal cell using a differential amplifier in each drivecell among a plurality of drive cells and switching among the pluralityof drive cells to drive the liquid crystal cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram of a liquid crystal cell driver inaccordance with the present invention.

[0013]FIG. 2 is a block diagram of another liquid crystal cell driver inaccordance with the present invention.

[0014]FIG. 3 is a block diagram of a display unit utilizing a switchingliquid crystal cell driver in accordance with the present invention.

[0015]FIG. 4 is a block diagram of a display unit utilizing a liquidcrystal cell driver in accordance with the present invention.

[0016]FIG. 5 is a flow chart illustrating a method of driving a displayin accordance with the present invention.

[0017]FIG. 6 is a timing diagram for static Vito in accordance with thepresent invention.

[0018]FIG. 7 is a timing diagram for a switched Vito in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In order to overcome the problems described above, it is proposedto add an amplifier such as a differential amplifier 16 between theinternal storage capacitance (14), and the LC cell (20) as shown inFIG. 1. In other words, a drive amplifier is added to the driving LCcell. This adds isolation between the storage capacitor and the LC cell.The added current drive capability ensures that the voltage on the pixelwill rapidly become that desired. It also allows for very low leakagecurrent from the storage capacitor (FET has very high input impedance),and allows for a continuous refresh of the voltage on the LC, whicheliminates the ‘droop’ problem, as well as the residual voltaicpotential stored in the cell. This should improve both the flickerissue, as well as the ‘image sticking’ problem which is associated withthe inability to achieve DC balance in the cell. It should also allowthe cell to work well even at somewhat elevated temperatures.

[0020] The disadvantage of this technique is that it increases the DCcurrent through the liquid crystal cell. This disadvantage can beovercome in part by gating the current source in the bottom of thedifferential amplifier. This can use the ‘pixel select’ or ‘row select’bit in the device (see FIG. 1). In this way, a periodic refresh of thevoltage can be achieved, while reducing the power consumption by 1/nrow,where nrow is the number of rows in the device. Since heating isuniform, this gating in some situations may not be needed.

[0021] A typical implementation in CMOS is shown in FIG. 1. Thecomponents are schematic representations, and alternate configurationscan be used without loss of generality. The key points are the amplifier16, which applies a closed loop correction voltage to the LC cell, andthe gated current source which allows reduction of power consumption.

[0022] Typically this circuit could be implemented with 3 transistors,which can be placed under the liquid crystal cell in an LCOS displaydevice. In the arrangement of FIG. 1, the amplifier 16 decouples the LCcell from the memory element. FIG. 1 illustrates a liquid crystal celldriver 10 for a liquid crystal display. The liquid crystal cell driverpreferably comprises a plurality of transistors (12, 15, 17, and 18)coupled to each other as shown in FIG. 1, a storage capacitance such asa storage capacitor 14, and a plurality of resistors 19 and 21.Preferably, three (3)transistors, such as transistors 15, 17 and 18 formthe amplifier 16, preferably in the form of a differential amplifierwhich serves as the buffer or isolation amplifier. The differentialamplifier 16 is preferably comprised of N-Channel transistors serving asan output to the liquid crystal cell. Additionally, the respectivesource electodes of the transistors of the differential amplifier aredriven by a current source such as an N-Channel transistor such astransistor 18 that may be gated. This current mirror arrangement ensuresa predetermined voltage on a given pixel. The differential amplifier 16is coupled between the storage capacitor 14 and provides isolationbetween the storage capacitor 14 and a liquid crystal cell or pixel.

[0023] The arrangement of FIG. 2 adds a global switch element (32) totransfer data from the storage element to the driver. This allows forincreased pixel drive for the same circuit operating voltages, andreduces image retention and flicker by allowing for inversion of thepixel drive voltage and the ITO transparent conductive electrode fromframe to frame. Referring to FIG. 2, another liquid crystal cell driver30 is shown similar to the liquid crystal cell driver 10 of FIG. 1. Inaddition to the elements previously recited with respect to cell driver10, cell driver 30 further comprises a global switch element 32 in theform of a transistor coupled between the storage capacitor 14 and thedifferential amplifier 16. The global switch element transfers data fromthe memory cell to a driver capacitor 36 of the display driver.

[0024] The problem of flicker has been addressed by many mechanisms inthe past. The issue of drive voltage and image retention has beenaddressed in digital drive LCOS displays. For analog systems, drivevoltage and image retention issues can be addressed similarly, due tothe need for continuous updating.

[0025] The main benefit of the technique disclosed herein is to separatethe driver capacitor 36 from the storage capacitor 14. This separationas shown in FIG. 2 allows for updating all the cells of entire LC arrayat one time if desired. The benefit of this is two-fold. First, thisseparation (with further pre-processing to determine which LC cells havechanged from frame to frame) also allows for updating of only the cellsin the LC array that have changed from a prior frame. In other words,the contents of the storage capacitor 14 can be changed withoutinstantaneously changing the display content on the LC cells. Thisgreatly reduces the data rate needed for static pictures. It also allowsfor the possibility of driving the display in an interlaced mode withoutdisplaying interline scanning artifacts. In a system which isinterlaced, normally odd lines are written on a first scan, and evenlines are written on a second scan. This scanning scheme produces theartifact known as ‘interline flicker’. This even happens for data whichis not intrinsically interlaced, such as film. The reason for “interlineflicker” is that the even lines from one frame are displayed at the sametime that the odd lines from the previous frame are displayed. Anyportion of the frame which changed will show an interline flicker. Withthe present invention, a display having an array of storage capacitors(14) corresponding to an array of LC cells would be updated just asnormal (even lines followed by odd lines), except that the entire LCarray would then be updated once the storage array has been filled.Thus, lines from different fames are never displayed simultaneously. Thesecond benefit of this technique is that it allows the common electrodevoltage to be modulated from frame to frame. This modulation increasesthe effective electric field which can be applied to the LC cell for agiven operating voltage of the driver circuit. This is a significantadvantage, as finer process geometry will reduce the maximum alloweddriving voltage. The benefits described above only occur when the pixelsare all updated at once. The simultaneous updating of the pixels canonly be done if the storage array (array of storage capacitors (14)) isseparated from the driver array (array of driver capacitors (36)).

[0026] The technique can most simply be implemented with the circuitdescribed in FIG. 1, with a modification as shown in FIG. 2. The voltageVnn of FIG. 2 is a static voltage which controls the current source forthe transistors 15 and 17. The Row and Column address are normaladdressing for an active matrix display. The control signals (Transferand Discharge) are separate globally controlled signals which transferthe charge on the storage capacitor 14 to the drive capacitor 36, whichdrives the LC cell. The additional transistors 32 and 34 and capacitor36 on the device are added to implement the new circuit andoperationally allows for the adequate discharge of current from thestorage capacitor 14 after each transfer. The additional componentsshould not be significant as process fabrication technology movesforward towards 0.1 microns and below.

[0027] In the arrangement of FIG. 1, an isolation amplifier was added todecouple the LC cell from the memory element, as explained before. Theadditional embodiment of the invention of FIG. 3, adds a second celldriver portion having a second storage cell (preferably storagecapacitor 14′) and amplifier (preferably differential amplifier 16′including transistors 15′ and 17′ and 18′) and a pair of transistors (72and 74) to switch between the two drive cells at a high rate of speed.The second drive cell also preferably comprises transistors 12′ and 18′and resistors 19′ and 24′ arranged similarly to the embodiment of FIG.1.) This eliminates flicker without the need for frame doubling. It canalso be used to increase the drive voltage available on the cell.

[0028] The basic advantage of the embodiment of FIG. 3 for driving LCOSis that it uses two separate storage elements and drive circuits thatare switched to drive the LC cell. This allows a fast switchingfrequency, which makes the flicker rate of the cell much abovefrequencies detectable by the human eye. It also allows for thepossibility of switching the common electrode voltage (Vito) to help toincrease the possible RMS voltage on the cell for a given operatingvoltage of the silicon back plane.

[0029] The upper cell (using transistor 72) contains the voltage todrive the LC during the ‘positive’ frame, the lower cell (usingtransistor 74) contains the voltage to drive the LC in the ‘negative’frame. The voltage during the positive and negative frames must bebalanced with Vito in order to avoid a net DC voltage on the cell, andresultant imager retention and reliability issues. VDD and VSS are theupper and lower operating voltages for the CMOS devices. VNN is set toregulate the current through the transistors of the differentialamplifiers, and controls the power dissipation of the amplifier. V1 andV2 are global switching voltages which determine which amplifier isdriving the Liquid Crystal cell. A timing diagram for a static Vito isshown in FIG. 6. A timing diagram for switched Vito is shown in FIG. 7.

[0030] Further referring to FIGS. 6 and 7, these timing diagrams reflectthe ‘positive’ picture data (V+) in the upper storage cell (14) in FIG.3, and the ‘negative’ picture data (V−) in the lower storage cell (14′).In the case of FIG. 6, Vito is not switched. When V1 is switched high(and V2 being low), the transistor 72 is turned on, and V+ is applied tothe LC cell. The effective voltage on the cell is (V+−Vito). At the nextswitching time, V1 is switched low, and V2 is switched high. Thetransistor 74 is turned on, and V− is applied to the LC cell. Theeffective voltage on the cell is (Vito−V−).

[0031] In the case of FIG. 7, Vito is switched. When V1 is switched high(and V2 being low), the transistor 72 is turned on, and V+ is applied tothe LC cell. Simultaneously, Vito is switched to low (Vito−), as shownin FIG. 7. The effective voltage across the LC cell is then (V+−Vito−).At the next switching time, V1 is set to low to turn off transistor 72,and switch V2 high to turn on transistor 74. This applies V− to thecell. Simultaneously, Vito is switched to high (Vito+), as shown in FIG.7. The effective voltage across the LC cell is then (Vito+−V−).

[0032] If the maximum value of V+ and V− is fixed by the maximum voltageof the backplane process, then a fixed value of Vito as in FIG. 6 mustbe (V++V−)/2. If Vito can be switched, as in FIG. 7, Vito− can be V−,and Vito+ can be V+. Thus, the maximum effective voltage on the LC cellis (V+−V−)/2 for FIG. 6, but V+−V− for FIG. 7. The timing of FIG. 7 canONLY be achieved on an analog system if all of the cells are updatedsimultaneously by the global switches V1 and V2.

[0033] Typically the time between V1 and V2 switching should be 1 or 2msec. This will need to be determined for a given set of LC materialsand the characteristics of the back plane, and response time of thedevices.

[0034] The obvious detriment of this scheme is that it requires a largenumber of transistors to implement (perhaps as many as 12). This shouldbe easily possible with a 20 micron pixel and a 0.35 micron process.Similar scaling on smaller pixels is possible with finer processgeometry.

[0035] Now referring to FIG. 4, a display unit 50 is shown that canutilize the display drivers 20 or 30 or 70 as previously describedabove. The display unit 50 preferably includes a plurality of displayelements arranged in a matrix of rows and columns and a memory elementand a liquid crystal cell. The driver preferably switchably outputs oneof a plurality of voltages to the display elements on at least one ofthe matrix of rows and columns, the display unit including aconventional decoder 51 and the driver controlled by the conventionaldecoder 51. The driver can include a storage capacitor and adifferential amplifier coupled between the storage capacitor and theliquid crystal cell, whereby the differential amplifier providesisolation between the storage capacitor and the liquid crystal cell. Thedriver can include a decoder and a plurality of semiconductor switchedcontrolled to be opened or closed by an output signal of the decoder 51.As shown in FIG. 4, the display unit 50 can include a row drive circuithaving a plurality of row (scanning) address lines 56 and a column drivecircuit 62 having a plurality of column (data) address lines 58.

[0036] Referring to FIG. 5, a flow chart is shown illustrating a method200 of driving a display in accordance with the present invention. Themethod 200 preferably comprises the step 202 of providing isolationbetween memory elements (such as a storage capacitor) and a liquidcrystal cell using a differential amplifier in each drive cell among aplurality of drive cells. Preferably, the isolation is provided betweena first storage capacitor and the liquid crystal cell using a firstdifferential amplifier in a first cell and between a second storagecapacitor and the liquid crystal cell using a second differentialamplifier in a second cell. The method 200 also preferably comprises thestep 204 of switching among the plurality of drive cells to drive theliquid crystal cell, where preferably a pair of transistors performs thefunction of switching between the first and second drive cells. Step 208can further provide the step of eliminating flicker without framedoubling. The method 200 may further comprise the step 216 of updatingan entire array of liquid crystal elements simultaneously and/or thestep 218 of updating only a memory cell that has changed from a previousframe. Additional benefits of the method 200 may include the step 220 ofdriving a memory array in an interlaced mode without displayinginterline scanning artifacts and/or the step 222 of modulating a commonelectrode voltage from frame to frame to reduce a required liquidcrystal drive voltage.

[0037] Although the present invention has been described in conjunctionwith the embodiments disclosed herein, it should be understood that theforegoing description is intended to illustrate and not limit the scopeof the invention as defined by the claims.

1. A display unit having an array of liquid crystal cells, comprising:an array of display drivers, a given display driver being associatedwith a given liquid crystal cell and including: a first storagecapacitance and a first amplifier selectively coupled between the firststorage capacitance and the given liquid crystal cell forming a firstdrive circuit; a second storage capacitance and a second amplifiercoupled between the second storage capacitance and the given liquidcrystal cell forming a second drive circuit; and a switching arrangementfor switching the first and second drive circuits to the given liquidcrystal cell.
 2. The display driver of claim 1, wherein the firstamplifier and the second amplifier are both differential amplifiers. 3.The display driver of claim 2, wherein the differential amplifiercomprises a pair of N-Channel transistors having respective drainelectrodes coupled and serving as an output to the liquid crystal cell.4. The display driver of claim 1, wherein the each of said first andsecond drive circuits further comprises a global switch element coupledbetween the respective storage capacitance and the amplifier, whereinthe global switch element transfers data from the storage capacitance toone of the first and second drive circuits.
 5. The display driver ofclaim 1, wherein the switching mechanism comprises a first transistordriven by a first global switching voltage and a second transistordriven by a second global switching voltage.
 6. A display driver for agiven liquid crystal cell of an array of liquid crystal cells,comprising: a first drive circuit coupled to a first memory element ofthe given liquid crystal cell; at least a second drive circuit coupledto a second memory element of the given liquid crystal cell, wherein atleast one of the first drive circuit and at least the second drivescircuit includes an amplifier; and a switching arrangement forrepeatedly switching the liquid crystal cell between the first and atleast the second drive circuits.
 7. The display driver of claim 6,wherein each of the first and the second drive circuits comprisedifferential amplifiers comprising a pair of N-Channel transistorshaving respective source electrodes coupled to a current source andserving as an isolation amplifier to the liquid crystal cell.
 8. Thedisplay driver of claim 6, wherein each of the first and the seconddrive circuits comprise differential amplifier comprising a pair ofN-Channel transistors having respective sources coupled to a currentsource.
 9. The display driver of claim 6, wherein the switchingmechanism comprises a first transistor driven by a first globalswitching voltage and a second transistor driven by a second globalswitching voltage.
 10. The display driver of claim 6, wherein thedisplay driver further comprises a global switching element which iscoupled between a storage capacitor and a differential amplifier andused for reducing image retention and flicker.
 11. The display driver ofclaim 6, wherein the display driver updates an entire array of liquidcrystal arrays elements simultaneously.
 12. The display driver of claim6, wherein the display driver updates only a memory cell that haschanged from a previous frame.